1. Field of the Invention
The present invention relates to a method of forming a multilayer resist pattern and, more particularly, to a method of forming, for example, a three-layer resist pattern (tri-level resist pattern) having a lower resist pattern formed of an organic material and an intermediate resist pattern substantially serving as an etching mask, capable of improving the resistance to ion bombardment of the intermediate resist pattern and of reducing critical dimension loss of the lower resist pattern.
2. Description of the Related Art
Demand for microprocessing techniques has been further enhanced with the shift of the design rule for designing semiconductor devices, such as LSIs, from a half-micron level to a quarter-micron level. Recent photolithographic techniques use exposure light having a short wavelength to enhance resolution, and requires multilayer resist processes because the recent semiconductor devices has a multilayer construction and the surface of the underlying layer on a substrate has large irregularities. The so-called three-layer resist process that uses a composite resist consisting of a comparatively thick lower resist layer capable of absorbing irregularities in the underlying layer and of forming a flat surface, a comparatively thin intermediate layer formed of an inorganic material and serving as a substantial mask pattern for etching the lower resist, and a sufficiently thin upper resist layer to achieve a photolithographic process in a high resolution is proposed by J. M. Morran and D. Maydan in J. Vac. Sci. Technol., 16, 1620 (1979).
In the three-layer resist process, the upper resist layer is exposed and developed in a desired pattern, the intermediate layer is etched by RIE (reactive ion etching) in a desired pattern by using the patterned upper resist layer as an etching mask, and then the lower resist layer is etched in a desired pattern by anisotropic dry etching using O.sub.2 gas and the patterned upper and the intermediate layers as an etching mask. This three-layer resist process is capable of forming a minute resist pattern in a high resolution over the irregular surface of an underlying layer.
Incidentally, when oxygen radicals (hereinafter referred to as "O*") are increased by increasing the gas pressure to etch the lower resist layer formed of an organic material at a high etching rate in a patterning process for patterning the lower resist layer by anisotropic etching using O.sub.2 gas, O* form undercut in the lower portion of the intermediate layer by isotropic oxidation to deteriorate the pattern of the intermediate layer.
On the other hand, etching conditions including low gas pressure and high bias power for increasing the mean free path of ions and substrate bias must be employed to carry out highly anisotropic etching and to prevent forming an undercut in the lower resist layer; that is, highly anisotropic etching is carried out by ion mode etching using the perpendicular incidence characteristic and the high kinetic energy of oxygen ions (hereinafter referred to as "O+") in combination. However, the employment of such etching conditions brings about the reduction of the etch selectivity between the intermediate layer and the underlying layer, which is one of the impediment to the practical application of the multilayer resist. This problem will be described with reference to FIGS. 1A, 1B and 1C.
FIG. 1A shows a workpiece provided with an upper resist layer 6 formed by a method of forming a second tungsten polycide wiring pattern in a SRAM fabricating process. A polysilicon layer and a tungsten silicide layer are formed sequentially over a stepped layer insulating film 2 so as to conform to the shapes of the steps to form a second tungsten polycide layer 3. In FIG. 1A, indicated at 7 is a first tungsten polycide gate wiring pattern and at 1 is a Si substrate. A device forming regions, a gate insulating film and the like are omitted. A lower resist layer 4 having a thickness large enough to absorb the steps of the second tungsten polycide layer 3 and to form a flat surface, an intermediate layer 5 formed of spin-on-glass (SOG) and a thin upper resist layer are formed in that order over the surface of the second tungsten polycide layer 3. The upper resist layer is patterned by a photolithographic etching process to form the upper resist pattern 6. Since the photolithographic etching process is applied to a flat surface, the upper resist pattern 6 can be formed in a high resolution. The pattern lines of the upper resist pattern 6 have sharp rectangular cross section having, for example, a width of 0.35 .mu.m. The intermediate layer 5 is patterned by a RIE process using the upper resist pattern 6 to form an intermediate resist pattern 5a as shown in FIG. 1B. The lines of the intermediate resist pattern 5a also have sharp shapes having a width of 0.35 .mu.m.
Then, the lower resist layer 4 is etched with O.sub.2 gas. The thin upper resist pattern 6 is etched out halfway through the etching process and the intermediate resist pattern 5a is exposed. Thereafter, the intermediate resist pattern 5a functions as a substantial etching mask. Since the lower resist layer 4 is formed in a thickness large enough to absorb the steps of the underlying layer for the purpose of the three-layer resist process, the lower resist layer 4 must be etched at a high etching rate. Therefore, the lower resist layer 4 is etched at a high etching rate by an ion mode highly anisotropic etching process using sputtering in combination. Since the intermediate resist pattern 5a of SOG serving as a substantial etching mask is formed at a comparatively low temperature of 200.degree. C. at the highest to obviate adverse thermal influence on the lower resist layer 4, the intermediate resist pattern 5a has a small density and a low resistance to ion bombardment. Therefore, the thickness of the intermediate resist pattern 5a and the width of the lines of the intermediate resist pattern 5a decrease with the progress of the etching process; that is, as shown in FIG. 1C, the edges of the lines of the intermediate resist pattern 5a are etched by a width x, causing pattern shift and, consequently, the width of the lines of the lower resist pattern 4a as finished is (0.35--2.times.) .mu.m, which is smaller than the design width of 0.35 .mu.m by a critical dimension loss of 2.times. and the width of the lines of the second tungsten polycide wiring pattern is reduced accordingly.